1. Field of the Invention
This invention relates to a method and apparatus for treating and processing a mixture of resins. More particularly, the invention relates to a new and improved method and apparatus for separating a mixture of ion exchange resins. The invention also relates to a new and improved method for cleaning a mixture of resins, particularly a mixture of ion exchange resins.
2. Description of Related Art
Ion exchange resins comprising a mixture of cationic and anionic resins are used in the production of high purity water. These resins are able to remove ionic impurities from water by a mechanism of selective ion exchange on a large number of active sites contained in the matrix of the resin. This process continues until such time as the active sites have been saturated with ionic impurities from the treated water. Commonly, at that time, the ion exchange resins are regenerated with highly concentrated ionic solutions to restore their capacity to remove the ionic impurities from water. It is generally preferred to separate the anion resins from the cation resins, before regeneration, so as to properly regenerate each resin.
Techniques to separate various types of synthetic ion exchange resins from one another have been used for decades around the world. The principal method of separating a composition of ion exchange resins which contains mixed cation and anion resins is via controlled hydraulic upflow in a vertical, cylindrical vessel. This technique is useful because the cation and anion resins are designed and manufactured with two different densities, thus allowing the cation resin to settle while the anion resin is lifted in a controlled upflow of water.
Mechanical engineering principles of fluid dynamics are used to affect the separation of the anion from the cation resins. Particle size differential in conjunction with density differences of the resin particles is used to optimize the degree of separation. An example to illustrate these properties, in which a resin mixture to be separated contains particles A and B, is as follows:
______________________________________ Cation Particle "A" diameter of 750 microns density of 1.23 g/ml terminal settling velocity of 0.09 ft/sec Anion Particle "B" diameter of 600 microns density of 1.08 g/ml terminal settling velocity of 0.03 ft/sec ______________________________________
In this example, it is clear than an upflow velocity of a fluid, such as water in a column, great enough to overcome the terminal settling velocity of Particle "B", but not great enough to overcome the terminal settling velocity of particle "A" results in a separation of the two particles, as particle "B" is carried up the column, and particle "A" falls toward the bottom of the column.
Although the use of separation processes involving upflow of water in a vertical cylindrical vessel has gained commercial acceptance, operating experience over the years has demonstrated that there are numerous deficiencies in the apparatus and processes used resulting in undesired effects, such as incomplete separation of the resins. Particularly, conventionally designed vertical, cylindrical vessels exhibit uneven hydraulic distribution and various dead-zones, (that is areas where turbulence of flow is insufficient to transport entrained particles) which adversely affect separation. These factors, along with the method of removing the resins from the separation vessels, result in varying degrees of material cross contamination, or unwanted mixing of particles, which results in incomplete separation of the anion and cation resins.
Conventional methods of separation further require the addition of large amounts of external water and require permanent installation at the site where the resins are to be processed. These factors are undesirable because they lead to increased costs and the production of a greater amount of waste water when external water is used.
U.S. Pat. No. 4,913,803 to Earls et al. is an example of a conventional particle separation apparatus and process. The process involves using a separation tube which has orifices so as to induce turbulent flow in the fluid flowing through the tube. The use of such orifices results in potential dead-zones in the separator and hence reduces the effectiveness of particle separation.
U.S. Pat. No. 4,120,786 to Petersen et al., is a further example of a known separation system which uses an upflow water stream. The separating column contains a pH monitor to determine the interface between the anion and cation resins and includes stacked baffles within the separation region. Such baffles can create dead-zones within the column which reduce the degree of separation of the cation from the anion resin particles.